In fuel cells, particularly in solid polymer electrolyte type fuel cells, ion-exchange membranes are used as solid polymer electrolytes and these electrolytic membranes are arranged both in order to intervene between anodes and cathodes. For example, hydrogen is supplied to the anode side, and oxygen or air is supplied to the cathode side, thereby allowing electrochemical reaction to occur to generate electricity.
As an electrode used in the solid polymer electrolyte type fuel cells, a so-called membrane electrode assembly (MEA) is known. In the MEA, electrodes are made of catalyst particles prepared by allowing carbon to support a noble metal, a solid polymer electrolytic component formed on the catalyst particles, and a fluorine resin adhering the catalyst particles to one another. The electrodes are each arranged on both sides of a solid polymer electrolytic membrane, thereby constituting a fuel cell (Japanese Publication of Unexamined Application, Kokai Publication No. 5-36418).
In some cases, both electrodes of anodes and cathodes in contact with the solid polymer electrolytic membranes contain noble metal catalysts such as platinum for enhancing the reaction. As for methods for producing the electrodes of this type, various methods have hitherto been proposed. Specifically, the electrodes are produced by coating ion-exchange membranes as the solid polymer electrolytic membranes, or gas diffusion electrode materials with mixtures of catalyst particles and solid polymer electrolytic components to form catalyst layers.
However, such connection of the catalyst layers with the solid polymer electrolytic membranes by coating or thermocompression bonding limits the reaction sites to two-dimensional interfaces between the electrolytes and the electrodes, so that the substantial working area is small. As one of approaches for improving this problem, it has been attempted that connections of an electrode material with a solid electrolytic membrane material are increased to make the three-dimensional reaction sites, thereby increasing the working area, by laminating an electron-ion mixed conductor layer comprising a mixture of carbon powder supporting a catalytic metal such as platinum, ion-exchange membrane powder and a polystyrene binder, on the ion-exchange membrane as the solid polymer electrolyte (for example, DenkiKagaku (Electrochemistry), 53 (No. 10), 812-817 (1985)).
In this case, a mixture of the carbon powder supporting a catalytic metal such as platinum, the ion-exchange resin (hereinafter also referred to as an "ion conductive component") and an organic solvent is usually mixed and stirred using a homogenizer or a ball mill to prepare a paste for forming an electrode catalyst layer (hereinafter also referred to as "electrode paste") (Japanese Publication of Unexamined Application, Kokai Publication No. 8-88008). Further, when the above-mentioned paste is adhered to a surface of a conductive, gas diffusion porous material or an ion-exchange membrane to form a catalyst layer, thereby preparing a gas diffusion electrode, it is required that the viscosity of the paste is increased for preventing the blocking of pores of a gas diffusion layer (Japanese Publication of Unexamined Application, Kokai Publication No. 8-236122). Then, when carbon paper is used as the porous material, a method of adding a thickner for adjusting the viscosity of the paste to 2,000 to 20,000 cp has been proposed (Japanese Publication of unexamined Application, Kokai Publication No. 8-236123).
Further, a method has also been proposed in which when carbon powder supporting colloid particles containing a catalytic metal such as platinum is crushed in a ball mill, the colloid particles containing a catalytic metal such as platinum partly adhere to fresh surfaces formed by crushing of the carbon powder to disperse the particles uniformly, and are partly converted to solid solutions or alloys by crushing energy to improve the catalytic activity (Japanese Publication of Unexamined Application, Kokai Publication No. 1-266848). Furthermore, when a catalyst in which particles of a metal such as platinum are supported on carbon is ground in a ball mill, the structure of carbon used as a support is broken to reduce the particle size and to homogenize the mixture of a catalyst and a binder (polytetrafluoroethylene) (Japanese Publication of Unexamined Application, Kokai Publication No. 5-47389). In addition, it has been known that when highly structured acetylene black having single particles linked to one another in chain form is ground by a grinding mill equipped with a cutter to form single particles (having an average particle size of 0.5.+-.0.2 .mu.m and an average specific surface area of 70 m.sup.2 /g or more), the dispersibility of platinum becomes higher than that of highly structured carbon to increase the catalytic activity, when it is used as a support (Japanese Publication of Unexamined Application, Kokai Publication No. 5-89880).